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Schmidt Newtonian Collimation
 

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Schmidt Newtonian Collimation and Care

 



 
 


Discussion will relate to the main telescope in use i.e. the Orion Optics 200 mm diameter f/4 Schmidt-Newtonian mounted on a Vixen GP equatorial base. The advantages and disadvantages of this combination have already been mentioned in my Author notes, suffice to say that weight and physical manageability are important factors for those past their prime. This telescope and others have been reviewed elsewhere together with Beginner's Advice (The Telescope Review Web Site)
The 200 mm diameter Schmidt-Newtonian telescope on the Vixen GP mount together with the telescope specification.

The accessories are stored in a secure (alarmed), dry and well ventilated room. The room houses a cupboard in which cables and other accessories are stored. The inside of the cupboard is maintained above 10 C, and more importantly above dew-point, through heating from a 40 watt electric bulb.

The following have all been found to be important in operation of a reliable system:

1. Following immediate use for storage, the telescope is carefully detached from the mount to avoid disturbance to collimation, the end cap is placed over the Schmidt corrector plate and the telescope wrapped in a thick blanket to ensure a slow warm up and change of temperature to avoid internal dewing.

2. The main mirror adjustment screws are screened by a custom made polystyrene cover, attached with Velcro tape, to avoid inadvertent movement during viewing. Remove during early evening thermal equilibration.

3. A simple dewcap some 300 mm in length made from black vinyl sheeting is imperative for many late evenings and nights.

4. The dewcap accommodates a (Hartmann) focusing mask at it's open end.  The mask made from rigid plastic (flooring tile) contains two diametrically opposite holes of 25 mm diameter with centres 155 mm apart. Focussing entails imaging a bright star (e.g. Vega) and racking in or out of focus until one small, very bright image results.

5. The dewcap can also accommodate at it's open end the coloured filter sheets (Imaging and Image Processing) for tri-colour imaging. The red, green and blue 250 mm diameter filter sheets are rigidly attached to plastic vinyl (floor tiling) frames to ensure flatness.

6. On cold, damp, nights the dewcap is inadequate and a Kendrick (Dew Zapper) heating tape, wrapped near the Schmidt plate is employed. The tape is supplied by a 15 volt dc power source delivering 2 amps.

The small focal ratio of the f/4 reflector offers a fast telescope with a wide field of view at prime focus of some 2.5 degrees. A standard Newtonian reflector would show serious coma problems towards the edge of the field, which it is the purpose of the Schmidt plate to correct. This it does usefully well but exposures on 35 mm film do still evidence some coma (tadpole images of stars) away from the central area. The coma has no impact on CCD images, where only the central portion of the light cone is being used on a CCD chip size of 4.9 by 3.6 mm. The small focal ratio can. however, make for difficult collimation, and loss of sharpness in images.

The collimation difficulty comes about because the geometric centre of the secondary mirror is offset, both away from the eye-piece and towards the primary mirror, to capture the cone of light from the primary. This offset ensures that the light axes of the primary mirror and the ocular will be coincident. The amount of offset can be calculated or measured from scale drawings of the light path, if primary diameter, focal length, distance from secondary to focal plane, and minor axis length of secondary, are determined with reasonable accuracy of a few percent. The collimation errors and calculation of the offset has been discussed by M.Bartels and others in some detail.

In my case the offset calculates as 3.9 mm. In order to ensure this offset, a cylindrical tube with cross-wires at one face was made from cardboard, which is slipped over the secondary mirror ( see J.B.Sidgwick, Amateur Astronomer's Handbook, p204, Faber and Faber Ltd., 1961). Reflected light from the primary is passed out through a side hole to a collimation tool (tube with cap containing a small central eye-hole and lower cross-wires) in the draw tube. The true centre of the secondary, indicated by the cross-wire reflections, could then be adjusted to the offset distance knowing the scale length of the cross-wires. Some patience is required here. All this is done following the equally important setting up of the primary in it's cell to be physically central and at right angles to the tube as mechanical measurement and placement can allow. Similarly, the secondary mirror is measured to be central,at first, in the Schmidt plate, and the mirror at nominal 45 degrees from vernier tool measurements.

The offset will be a considered requirement for any reflector under say f/6 focal ratio. It means that the reflection of the secondary in the primary is not concentric with the centred primary mirror, but will be shifted to the right, if the primary is right of the draw tube, when viewed from the draw tube. On the other hand, the reflection of the draw tube end and the collimation tool hole will and should be located in the centre of the primary mirror refection and ensure the collimation is complete. Click here for illustration of collimated view through the focusser tube. All this is necessary, since a diffraction ring star test is difficult to achieve. Even with good seeing, it is difficult to reach sufficient magnification to detect a fine ring structure. This has been true for a 6 mm orthoscopic eye-piece and 2 times Barlow i.e. magnification of 267. Racking in and out around the focus to check for eccentricity of the image is about as much as one can do as a collimation check. The real check is a target, such as the Moon or a planet, to see the crispness of the image. After a while, one gets a feel for just how poor or good it is, and whether to tweak the primary mirror screws, the positions of which have been marked, as a precaution.
Futher Study

Collimation of Schmidt Cassagrain telescopes can be equally complex and has been examined by others ( Optical Collimation of Telescopes).

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